tunnelling and underground space technology

Tunnelling and Underground Space Technology 24 (2009) 466–471

Contents lists available at ScienceDirect

Tunnelling and Underground Space Technology

journal homepage: www.elsevier .com/locate / tust

Trenchless Technology Research

Experimental study on the effect of injecting slurry inside a jackingpipe tunnel in silt stratum

Zhou Song a,b, Wang Yingyi a, Huang Xingchun a,*

a Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, PR Chinab Shanghai No. 2 Municipal Engineering Co. Ltd., Shanghai 200030, PR China

a r t i c l e i n f o

Article history:Received 1 October 2007Received in revised form 29 October 2008Accepted 3 November 2008Available online 27 December 2008

Keywords:Silt stratumPipe jackingEffect of injecting slurryModel test

0886-7798/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.tust.2008.11.003

* Corresponding author.E-mail address: huangxc@sjtu.edu.cn (X. Huang).

a b s t r a c t

Through a three-dimensional model test, the variations of jacking force and ground settlement inside ajacking pipe tunnel in silt stratum, under testing conditions which include non-injection and differentsynchronized injections of slurry, are simulated in order to study the effect of traditional thixotropicslurry (consisting of bentonite, CMC (Carboxymethyl Cellulose) and soda ash) and HL compound slurryto decrease frictional resistance and ground settlement. And then their mechanical properties are inves-tigated according to the different slurry micro-structure features which are captured by an electronmicroscope. The tests and analysis demonstrate that: (1) injecting thixotropic slurry inside a jacking pipetunnel has conspicuous effects in reducing frictional resistance, and (2) viscosity and condensation forceof the slurry have a dominant effect on decreasing frictional resistance and ground settlement, respec-tively. To reach optimal jacking distance and efficiently control ground settlement, more attention shouldbe given to slurry parameters in the actual engineering processes. The research results indicated excellenteffects when they were applied to the jacking pipe of Shanghai Lingang New City Sewage Conduit.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Up to now, many studies on the variations of jacking force andground settlement, which are extruded inside a jacking pipe tun-nel, have been done by means of laboratory tests and engineeringpractice as well as theoretical analysis to investigate the effect ofslurry on decreasing frictional resistance and ground settlement.These studies have utilized pipe jacking in clay stratum (Chapman,1999; Marshall, 1998; Barla et al., 2006; Chapman and Ichioka,1999; Pellet-Beaucour and Kastner, 2002; Ding, 2003; Cao andWu, 2005; Wei et al., 2004; Feng et al., 2003; Qiao and Deng,2000; Luo and Zhou, 2003; Fang et al., 2003; Fang and Weng,1998). However, as is already known, the effect of thixotropic slur-ry is quite different due to the complexity of geological conditions,groundwater and the microstructure of soil. For silt stratum, as itexhibits loose soil structure, low binding power and susceptibilityto groundwater, both condensation force and viscosity of the slurrywill inevitably be affected. Thus the effects of decreasing frictionalresistance and ground settlement are also impacted. During thelast few years, there has been hardly any systematic research inthis field, and little is known about the results on the effect ofthixotropic slurry in silt stratum. From the above-mentioned re-search results, the actual effects in the field of a jacking pipe tunnelin silt stratum have not yet been sufficiently analyzed.

ll rights reserved.

In this paper, based on the pipe jacking of Shanghai LingangNew City Sewage Conduit, the construction process was simulatedby remodeling a sampling of silt stratum in situ. Thus the effectand mechanical properties of decreasing friction and ground settle-ment of thixotropic slurry, which have different physical andmechanical properties, are studied in pipe jacking conduit. In addi-tion, the experimental study results could be directly applied to ac-tual engineering processes. Thus, several theoretical and practicalfindings have been made.

2. Description of test system and method

2.1. Test system and measured point distribution

The test system is composed of a test chamber, load system,measurement system and injection system, as shown in Figs. 1–3.

2.1.1. EquipmentsChamber: open upper surface and sealed other surfaces; PVC pi-

pes(inner diameter: 100 mm, thickness: 5 mm):1 with 2000 mmlong, 12 with 120 mm long among which 6 pipes had 4 � 5 mm in-jected holes, numbered A–F.

2.1.2. Load systemHydraulic jack: the rated lifting capacity is 3t, rated hoisting

height is 140 mm; reaction planes fixed on the ground.

Static Strain GaugeComputer

Slurry pipe Soil

Soil

Test chamberReaction Frame

Jack

Reserve pulp barrel

Serious shunt

Pressure pump

1501.5 1.5

401.

550

1010

Pressure sensor

Fig. 1. Abridged general view of test system (dm).

110

90

jacking pipe tunnel

1597

0 5

15 1470 15

190

S1

S2

S3

S4

S5

400

6024

025

014

0

Fig. 2. Planform of test model and measured point (cm).

Fig. 3. Panorama of test system.

Table 1Soil’s characteristics.

Name Watercontent(w%)

Specificgravity(Gs)

Dry unitweight(c dkN)

Voidratio(e0)

Saturation(Sr%)

Cohesion(C kPa)

Internalfrictionangle (�)

Sandy silt 28.70% 2.70 14.60 0.81 95.003.00 30.80

S. Zhou et al. / Tunnelling and Underground Space Technology 24 (2009) 466–471 467

2.1.3. Measurement systemHorizontal displacement gauges, which are equidistantly laid in

the position of 400 mm, 100 mm *110 mm box section from theupper section of the space (No. S1–S5 in Fig. 2 respectively); pres-sure sensors; 50 channel static resistance strain gauge; automaticdata acquisition system.

2.1.4. Injection systemSlurry barrels; injecting pipes; pressure pump; pressure gauge;

plastic pipes with 5 mm in diameter.

2.2. Description of test method

The soil samples in the test, taken from the Shanghai LingangNew City Sewage Jacking Conduit, were in silt stratum on site. Inorder to simulate the conditions of the construction site, and alsomake several tests comparable, the soil samples were screened be-fore placing them into a box, then reshaped and saturated by add-ing water to the soil, so that the density of soil in silt stratum wasthe same as that of the undisturbed soil. The test methods antici-pated and encompassed the variations in construction site condi-tions at various tunnel locations.

To simulate the annular gap between the tunnel structure andthe soil at the actual construction site, a stainless steel ring with5 mm thickness was fixed in the forefront of the pipe.

The silt filling the box was separated to 5 layers, each with20 cm in depth, and each layer was compacted to consolidate.When the second layer was filled, the 2000 mm long pipe penetrat-ing the whole model box was inserted into the box and then filledthe box completely with silt. The steps listed above were repeatedbefore each test.

The No. A short pipe with injected holes was linked to the longpipe box through stainless steel ring. A hydraulic jack was placedin the posterior of the No. A short pipe. And it started to jack pipeat the rate of 1cm/5min when the jacking force sensors and dis-placement sensors were properly arranged. At the same time, thedata acquisition system was activated to collect data.

One short-pipe without injected holes was linked with No. Apipe when No.A pipe had completed jacking, then simulation ofan injecting system was pulsed on. The steps listed above were re-peated throughout the entire trial. Two types of slurry were in-jected at the same pressure through the same period to makesure the injected volume be equal. Soil’s characteristics are shownin Table 1.

3. Analysis of test results

3.1. Effects of decreasing friction by injecting

In order to study the effect of injecting slurry in silt stratum, thedistribution of jacking forces in the construction process was sim-ulated in different operating modes, such as injecting traditionalslurry, injecting HL compound slurry and without injecting slurryat all. The composition parameters of thixotropic slurry are shownin Table 2.

Table 2Components’ ratio of thixotropic slurry.

Slurry type Ratio of components

Bentonite CMC Soda Water Polymer

Traditional slurry 1 0.012 0.05 5.999 0HL compound slurry 1 0.012 0.05 5.999 0.05

468 S. Zhou et al. / Tunnelling and Underground Space Technology 24 (2009) 466–471

In order to facilitate comparative analysis, the relation betweenjacking force ratio and jacking distance ratio, under the conditionsof non-injection slurry and injecting different kinds of slurry, issummarized in Fig. 4.

It is known that the jacking force ratio distribution curve with-out injecting slurry has the following characteristics: Firstly, thejacking force increases as the jacking distance increases becausethe effective length of the pipe generally increases continuously.Secondly, the jacking force is discontinuous at certain locations,which, for example, occurred in one control section of the dockingsocket. At this point the jacking process would stall, regressing to alower rate, and then it increases due to driving of the jacking pipe.The jacking force would suddenly increase when new pipe is setand the hydraulic jack continues to start again, but it wouldquickly return to the stable value after a very short time.

When injecting traditional slurry, the jacking force would in-crease with increasing jacking distance. The jacking force is alsodiscontinuous at docking new pipe sockets. However, the distur-bance of soil around the pipe would be extremely small becauseof injecting thixotropic slurry in the gap between the pipe andthe surrounding soil. The thixotropic slurry could play a importantrole in easing the effect of interaction between them. Thus, thejacking force value in this test would be less than that withoutinjection.

Similar to that of traditional slurry, while injecting HL com-pound slurry, the jacking force would increase with increasingjacking distance. The jacking force is also discontinuous at dockingnew pipe sockets. But the jacking force value is noticeably largerthan with that with injecting traditional slurry.

It has also been shown that all of the jacking forces are consis-tently constant when the jacking distance is less than some valueat left of short pause (the ratio of jacking distance to jacking dis-tance maximum is approximately 0.466) shown in Fig. 4. The rea-son is that the shield formed by slurry gradually makes thefrictional resistance decrease.

In addition, in order to investigate the change of the jackingforce and time-dependence of slurry effect, the jacking pipe is

0.0 0.2 0.4 0.6 0.8 1.0-0.2

0.0

0.2

0.4

0.6

0.8

1.0 No slurry Traditional slurry HL compound slurry y=0.00231exp(x/0.20117)+0.58554 y=0.27692exp(x/0.97548)-0.06290 y=0.01762exp(x/0.31452)+0.00754

F/F m

ax

D/Dmax

Pause Jacking for 24 hours

Fig. 4. Comparison of Jacking force in 3 tests.

paused for 24-h at the place shown in Fig. 4 and then the hydraulicjack is restarted for jacking again. It is shown that the jackingforces suddenly increase under both conditions of injecting HLcompound slurry and no slurry. The reason is that the slurry trans-forms from solution to gel due to its thixotropy during the pauseperiod. Thus, the reduced friction effect of thixotropic slurry de-creases during this process. As a result, the jacking force suddenlyincreases by 66%.

In contrast with the jacking force, under the condition of inject-ing traditional slurry, the jacking force maximum value decreasesby 47.8%, the average value decreases 78.6%. Under the conditionof injecting HL compound slurry, its maximum capacity decreasesby 25%, and the average value decreases by 15%.

It is shown that all of the injections of thixotropic slurry cangreatly decrease the frictional resistance of the jacking pipe tunnelin silt stratum according to the above experimental results. Anddecreasing frictional effect and behaviors of thixotropic slurry havevery large differences with each other. If one only wants to de-crease friction, traditional slurry is better than the HL compoundslurry. The study of slurry micro mechanism would explain itbelow.

The changes of friction per unit area along jacking distance ofmodel under three experimental conditions are shown in Fig. 5.

In all three experimental conditions, the friction ratio per unitarea gradually decreases in negative exponential type. Whenincreasing jacking distance ratio, the friction ratio would graduallybecome more stable under three experimental conditions: inwhich that of no injection stabilizes in 0.18, that of injecting tradi-tional slurry stabilizes in 0.07, and that of injecting HL compoundslurry is more or less 0.09, as shown in Fig. 5.

It appears that traditional slurry, in pipe jacking conduit in siltstratum, plays a more significant role than HL compound slurryin terms of reduced frictional resistance.

3.2. Control soil settlement by injecting

Under three different experimental conditions, the relationshipbetween soil settlement and jacking distance is shown in Fig. 6(comprised of three graphs). Among them, (a) non-injection; (b)injecting traditional slurry; (c) injecting HL compound slurry. Asshown as Fig. 6:1) at the beginning, the soil settlement is small un-der the different conditions, it starts to rise up while the distancebetween the trim section and settlement monitoring line de-creases, it then becomes stable after the distance ratio reaches0.4. 2) It can be shown in the Figs. 6a–6c that the settlement values

0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.2

0.4

0.6

0.8

1.0 No slurry Traditionalslurry HL complex slurry y=2.42112exp(-x/0.11669)+0.17996 y=1.62627exp(-x/0.13076)+0.08804 y=0.00005exp(-x/0.14442)+0.03954

f/fm

ax

D/Dmax

Fig. 5. Comparison of Average friction of unit area in 3 tests.

0.0 0.2 0.4 0.6 0.8 1.0-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

y=-0.167+0.165/(1+exp((x-0.189)/0.164)) y=-0.190+0.178/(1+exp((x-0.281)/0.154)) y=-0.458+0.374/(1+exp((x-0.186)/0.121)) y=-0.544+0.487/(1+exp((x-0.275)/0.119)) y=-0.971+0.856/(1+exp((x-0.257)/0.089))

s/s m

ax

D/D max

S1

S5

S2

S4

S3

Fig. 6a. Ground settlement under the condition of no injection.

s/s

0.0 0.2 0.4 0.6 0.8 1.0-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

y = 0.067 -0.029/(1 + exp((x+1.288e19)/-1.464e23)) y =-0.078+0.220/(1 + exp((x+0.166)/0.203)) y =-0.170 +0.207/(1 + exp((x-0.346)/0.031)) y = -0.345+0.294/(1 + exp((x-0.305)/0.071)) y = -0.671+0.546/(1 + exp((x-0.299)/0.049))

S3

Mea

surin

g Li

ne

max

D/Dmax

S5

S2

S1

S4

Fig. 6b. Ground settlement while injecting traditional slurry.

s/s

0.0 0.2 0.4 0.6 0.8 1.0-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

y =-0.010+0.012/(1 + exp((x-0.393)/0.026))

y =-0.066+0.065/(1 + exp((x-0.270)/0.054))

y =-0.161+0.176/(1 + exp((x-0.312)/0.124))

y =-0.158+0.162/(1 + exp((x-0.276)/0.065))

y =-0.259+0.263/(1 + exp((x-0.289)/0.067))

max

D/Dmax

Mea

surin

g Li

ne

S5S2

S1

S4

S3

Fig. 6c. Ground settlement while injecting HL compound slurry.

Fig. 7. Microscopic composition of slurry.

S. Zhou et al. / Tunnelling and Underground Space Technology 24 (2009) 466–471 469

under the three experimental conditions differ significantly: themaximum of soil settlement ratio is �1.0 without injection, it is�0.7 with injection of traditional slurry and it is �0.3 with injec-

tion of HL composite slurry. In the case of monitoring point S3,in contrast to the settlements without injecting, it is decreasedby 68% with injecting traditional slurry, and it is decreased by75% with injecting HL compound slurry. HL compound slurry hasa better effect to control settlement than traditional slurry.

As described above, the jacking force and soil settlement arecharacterized by: (1) jacking force control and settlement controlare very unfavorable without injection; (2) in reducing frictionand controlling soil settlement, traditional slurry and HL com-pound slurry have both merits and demerits. In the pipe jackingin silt stratum, the traditional slurry is conducive to reduce friction.However, HL composite slurry is better suited to control settle-ment. Selection is done according to the specific project needs.

4. Analysis of micro mechanism

Silt stratum has the following features: loose structure, satura-tion, low bond strength and large internal friction angle, so thatsoil is directly linked with the outer surface of the pipe, increasedjacking frictional resistance appears without injection. Whileinjecting, a slurry jacket is formed in the gap between slurry pipeand soil, it serves to lubricate and support the soil.

In order to analyze the applicability and mechanical propertiesof different slurries in silt stratum, the microstructure of slurry issummarily analyzed. According to the trial results extracted fromthe literature (Ge and Zhang, 2005), electron microscopic struc-tures of surface and section of traditional slurry and HL compoundslurry are shown in Fig. 7.

10

20

30

0Dynamic shear (YP)

Traditional slurry HL Compound slurry

Surface viscosity (AV) Static shear (G10)

Fig. 8. Comparison of Condensation force and viscosity of two slurries.

-200 0 200 400 600 800 1000 1200 1400 1600 1800-5

0

5

10

15

20

25

North Line Tunnel South Line Tunnel

Jacking Distance, m

Aver

age

frict

ion

of u

nit a

rea,

KPa

Fig. 10. Average friction per unit area vs. jacking distance in-situ.

0 10 20 30 40 50 60 70 80-60-55-50-45-40-35-30-25-20-15-10

-505

10152025

y=-3.98257+4.01114/ (1+exp((x-17.20953)/1.04194))

y=-5.68753+5.74287/ (1+exp((x-14.11026)/3.42958))

y=-12.84174+13.80006/ (1+exp((x-10.44412)/4.71637))

y=-13.67332+14.73749/ (1+exp((x-10.61417)/4.9347))

y=22.31763-20.68415/ (1+exp((x-12.55832)/3.04089))

B13B14

B12 B11

Settl

emen

t, m

m

Time, day

B106m 6m 6m 6m12

m

Tunnel

B13B11B10B12B1 4

Fig. 11. Relationship between ground settlement and time in-situ.

470 S. Zhou et al. / Tunnelling and Underground Space Technology 24 (2009) 466–471

The results show that the density of HL compound slurry is sig-nificantly higher than that of traditional slurry. Therefore, HL com-pound slurry has a comparatively better suspension supportingeffect in silt stratum. Ground settlement is controlled more effec-tively in macro.

The test results of viscosity and condensation force of twomaterials are shown in Fig. 8.

Due to added polymer materials, in contrast with traditionalslurry, the surface viscosity of HL compound slurry is greater, dy-namic and static shear strength are also greater. The friction perunit area between HL compound slurry and pipeline is greater thanthat of traditional slurry, thus the effect of HL compound slurry toreduce friction resistance is less than traditional slurry.

As mentioned above, in the jacking pipe projects, the frictionresistance reduction per unit area depends on the surface viscosityof slurry, static shear stress and dynamic shear stress; slurry den-sity has the big impact on controlling ground settlements.

5. Analysis of application to engineering

To verify the validity, test results are compared with measureddata in the pipe jacking of Shanghai Lingang New City Sewage Con-duit. The relationship between jacking force and jacking distancein the construction site is shown in Fig. 9, and the relationship be-tween friction resistance per unit and jacking distance is shown inFig. 10.

In general, the jacking force would increase while jacking dis-tance increases, the measured results correspond to the test re-sults. It is consistent with the test results that jacking forcebecomes discontinuous at about 1100 m set because of stoppingthe jacking machine for materials. As shown in Fig. 10, friction

0 200 400 600 800 1000 1200 1400 1600 18000

500

1000

1500

2000

2500

3000

3500

North Line Tunnel South Line Tunnel

Jacking Distance, m

Jack

ing

forc

e, T

Jump point of jacking force

Fig. 9. Relation between jacking force and jacking distance.

resistance per unit is stable during the jacking process, from1 kPa to 2 kPa. This is consistent with test results

The changes of the settlements in actual operating conditionsare very similar to that in the test, as shown in Fig. 11. The moni-toring positions were placed 10m above one house (numberedB10–B14), and the settlements were measured since the workingface was 20 m behind the monitoring section. At the beginning ofthe pipe-jacking, the ground exhibits slight uplift, therefore, settle-ments occur in the first five days after jacking, then the settlementsvary significantly and the rate of settlement increases, settlementchanges rapidly, that is to say, the change rate is greatest in theperiod of 10–20 days. The rate of settlement becomes less after20 days, settlement is stable in 30 days, and the maximum reaches23 mm.

6. Conclusions

Engineering monitoring results and test results show that:

(1) Injecting thixotropic slurry behind pipe wall has a significanteffect of reducing friction resistance and controlling groundsettlement.

(2) High density slurry could better support soil and reduce thesurface settlement, due to looseness and low cohesion andalso saturation of soil in silt stratum.

S. Zhou et al. / Tunnelling and Underground Space Technology 24 (2009) 466–471 471

(3) Based on having formed a slurry jacket, the frictional force isdirectly controlled by surface viscosity and gel force ofslurry, then the jacking force is controlled by them.

(4) Due to thixotropy, slurry exhibits sol–gel transformationafter a period of stationary standing, which minimizes itseffect on reducing friction resistance, as a result, the jackingforce would suddenly increase.

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